The invention relates generally to micro-electromechanical systems (MEMS) or micro-optical-electromechanical systems (MOEMS). More particularly, the invention is directed to a micro-electromechanical hinged flap structure.
MEMS and MOEMS systems (hereafter xe2x80x9cMEMSxe2x80x9d) combine electronics with micro scale mechanical devices, resulting in microscopic machinery, such as sensors, valves, gears, mirrors, and actuators embedded in semiconductor chips. The MEMS manufacturing process is similar to that used in the semiconductor industry, wherein silicon wafers are patterned via photolithography and etched in batch processes.
Most MEMS devices are limited to two-dimensional (hereafter xe2x80x9c2Dxe2x80x9d) structures; that is, structures that appear in two dimensions, typically in a horizontal plane, with limited extension to a third dimension, typically a vertical plane. However, three-dimensional (hereafter xe2x80x9c3Dxe2x80x9d) MEMS structures may be desirable for some applications, particularly in optical systems on a chip where 3D micro-optical components are needed. Examples of 3D micro-optical components include laser collimators, tunable lasers, tunable filters, beam steering reflectors, corner cube reflectors, and tunable Fabry-Perot etalons. Assembly processes have, therefore, been developed to construct 3D microstructures from 2D shapes formed using basic MEMS manufacturing technology.
In general, such assembly methods are performed on devices either serially, i.e., assembling one component at a time, or in parallel, i.e., assembling multiple components simultaneously. 3D assembly may also occur once or multiple times during manufacture, as well as singly or in a batch process. Batch assembly is typically preferred since it retains the economic efficiencies inherent in batch MEMS fabrication techniques.
Furthermore, the assembly process may be manual, such as by placing a vertical component directly onto a horizontal substrate, or automated where a 3D structure is self-assembled with minimal human intervention. Manual assembly is generally restricted to the research and development environment, since it is a time-consuming and low-yield procedure, whereas automated assembly is generally reserved for a high-yield manufacturing environment. Automated self-assembly is suitable for batch assembly in a manufacturing environment, particularly for on-chip systems that have a high density of 3D structures, which are to be assembled in parallel. An example of such a 3D automated assembly process is disclosed in U.S. Pat. No. 6,166,478 to Yi et al. (hereafter xe2x80x9cYixe2x80x9d), which is incorporated herein by reference.
Likewise, automated self-assembly of 3D MEMS structures can be performed by powered or powerless assembly processes. Powered assembly forms 3D structures by consuming external energy in such forms as electrostatic force (see Bibliography 1), magnetic force (see Bibliography 2), Lorentz force (see Bibliography 3), thermal bimorph (see Bibliography 4), and fuel that powers a microengine (see Bibliography 5). Powerless assembly assembles the 3D structures by applying a specific material to the structures and inducing deformation or surface tension of the specific material. Common materials used for the powerless process include solder6, polymer7, etc. Examples of the abovementioned powered and powerless assembly processes can be found in the references listed in the bibliography section below.
In addition, both hinged and hingeless structures are able to be self-assembled to form 3D structures. An example of a hinged structure can be seen in Yi, which teaches a method for assembly of MEMS systems using magnetic actuation. FIGS. 1A and 1B are isometric views of prior art MEMS systems 100 and 120 shown and described in the Yi patent. Yi teaches a MEMS system that includes at least two hinged flaps, namely a primary flap 102 or 122, and a secondary flap 104 or 124. Each flap includes a different amount of Permalloy or other magnetic material 106 or 126 affixed thereto. Furthermore, the flaps are hinged to a substrate at hinges 108 and 110, or 130 and 132. The flaps are hinged at an angle to each other, and are rotated out of the plane of the wafer substrate when placed in a magnetic field of sufficient strength. When placed in an increasing magnetic field, the flaps are raised asynchronously, at different times, due to the different amounts of Permalloy used in the flaps. As they are raised, the flaps interact with each other and become interlocked, as shown in FIGS. 1A and 1B.
Due to the nature of MEMS manufacturing, shafts of any hinge 108 and 110, or 130 and 132 are typically square or rectangular in cross-section; see FIG. 6. This necessitates creating large clearance spaces within the hinge to allow the shaft to rotate. Each shaft is attached to, or forms part of, each rotatable flap. However, these large clearance spaces lead to flaps that are unstable and that can shift in their hinges in both X 112 and Y 114 directions. This dramatically affects the hinge tolerances, thereby negatively affecting the tolerance of the primary flap with respect to the vertical. In typical applications, it is desirable that the primary flap be as close to vertical as possible, such as within 0.03 degrees. However, with the above described system, if a secondary flap shifts as little as 3-5 micrometer longitudinally within its hinge, and the distance between where the secondary flap is connected to the primary flap and the secondary flap""s hinge is 300 micrometer away (vertically), the angular error of the primary flap to the vertical can be as much as arctan(4/300)=0.76 degrees.
Moreover, once assembled, the secondary flaps 104 or 124 interlock with the primary flap 106 or 122 to hold the primary flap in an assembled position. The secondary flaps 104 or 129 are, however, substantially free to rotate away from the primary flap 106 or 122 and, thereby, release the primary flap from their interlocked position. The only way the primary flap is held in position is by the use of scissor hinges, which use a cantilevered staple to exert a retaining force on the primary flap. A detailed description of scissor hinges can be found below in relation to FIG. 6. The scissor hinges, however, do not provide enough force on their own to secure the primary flap in position, hence the use of the secondary flaps. Furthermore, scissor hinges alone cannot ensure that the primary flap is vertical enough. The vertical angle of the primary flap is basically constrained by the etched edge of the primary flap and the etched surface of the substrate. In addition, the scissor hinge only secures the bottom of primary flap. Air damping or shock may push the primary flap back to the horizontal plane.
Moreover, the secondary flaps are only held in position by friction forces between the secondary flaps and the primary flap. This leads to an untenable position where should the system undergo a shock, such as by being bumped, the secondary flaps will rotate away from the primary flap, allowing the primary flap to rotate out of its vertical position, thereby destroying the 3D structure. Therefore, the system disclosed in Yi does not provide a suitable solution for securing secondary flaps into an accurately controlled position in a 3-D MEMS structure.
In light of the above, a need exists for a 3-D MEMS structure that includes a system of flaps that can be assembled through automated self assembly, while permanently and accurately maintaining the positions or angles of the flaps.
To address the drawbacks associated with the prior art, a micro-electromechanical hinged flap system is provided. The micro-electromechanical hinged flap system includes a substantially horizontal substrate and a main flap hinged on one side thereof to the substrate. The system also includes at least one locking flap, preferably two, for securing the main flap in a substantially vertical position. The locking flap is coupled to the substrate by means of a biasing mechanism that continually forces the locking flap toward a position parallel to the substrate. The biasing mechanism is preferably a torsion bar or a bending mode spring, i.e., a mechanical spring, such as a cantilevered arm. The main flap is preferably hinged to the substrate along a hinge axis that is substantially perpendicular to a rotational axis of the locking flap.
Also provided is a method for assembling a micro-electromechanical hinged flap system. The method starts with an unassembled and relaxed 2D system that mechanically responds to an external source, such as a magnetic field or a change in temperature. A locking flap is rotated through an acute angle against a biasing force. The biasing force is caused by a biasing mechanism coupling the locking flap to a substrate. A main flap is then raised, whereafter the locking flap is released, such that the biasing force causes the locking flap to engage with the main flap, thereby securing the main flap in position at the predetermined angle.
Externally applied forces induce torque forces on the main and locking flaps, rotating all the flaps to the desired positions and angles. This self-assembly process can be used in batch assembly or individual assembly for 3D micro-structures on a chip or wafer. Once the systems are assembled by this process, they can sustain their positions or angles permanently, or they can be disassembled to their original 2D form.
The biasing force of each locking flap applies equal and opposite biasing forces to opposite sides of the main flap, preventing lateral motion of the main flap. Also, once fully engaged, alignment guides on each locking flap prevent lateral motion of the main flap. Therefore, locking flaps together with a hinge coupling the main flap to the substrate, substantially increase the stability of the main flap in the 3D structure.